Generally, a fuel cell is formed by stacking a membrane electrode assembly that forms a catalytic layer and a gas diffusion layer on each of the surfaces of an electrolyte membrane and interposing a separator therebetween. WO 01/017048 discloses an electrolyte membrane exposed to the catalytic layer. The gas diffusion layer is not formed at an outer peripheral portion of the membrane electrode assembly. Further, a gasket formed with a resin material for providing sealing between the outer peripheral portion and the separator is applied thereto. An optimum thickness of such a gasket is determined based on a thickness of the gas diffusion layer. Thus, when there is a change in size of the gas diffusion layer, it is necessary to prepare various types of gaskets with different thicknesses to accommodate the changing thickness of the gas diffusion layer. However, gaskets of varying thicknesses are limited, and it is possible that a gasket having the optimum thickness for a gas diffusion layer cannot be used. For example, when the gasket is not thick enough, the gas diffusing performance and the drainage performance deteriorate since the gas diffusion layer is excessively pressed and crushed. Alternatively, when the gasket is too thick, the power generating efficiency deteriorates since the electrical resistance of the gas diffusion layer increases.